Analog-to-digital converters have seen increased use in recent years due to the advances in digital signal processing and the increased use of digital transmission systems. Typically, analog-to-digital converters comprise circuitry for receiving an analog input signal and outputting a digital value that is proportional to the input analog signal. This digital output value can either be a parallel word or a serial digital bit string. There are many types of analog-to-digital conversion schemes such as voltage to frequency converters, charge redistribution, delta modulation, etc. Each of these techniques has advantages and disadvantages associated therewith.
One type of analog-to-digital converter that has seen increased use in recent years is that utilizing delta-sigma modulation wherein an analog voltage is input to a delta-sigma modulator and the output thereof filtered to remove the noise. The delta-sigma modulator is of the type which converts an analog input to a digital pulse string having an average amplitude over time proportional to the analog input. One type of delta-sigma pulse modulator is described in U.S. Pat. No. 4,542,354, issued Sept. 17, 1985 to Robinton, et. al. Delta-sigma modulation provides for high accuracy and wide dynamic range, as compared to the earlier delta modulation techniques. The delta-sigma type modulation is sometimes referred to as an oversampled converter architecture which is immune from some of the earlier undesirable second order effects of delta modulation.
There are two key components of a delta-sigma analog-to-digital converter, the analog modulator and the digital filter. The analog modulator oversamples the analog input and produces a low-resolution digital output. However, with any A/D converter, there are a number of noise sources that are inherent to any analog modulator design. In a delta-sigma modulator, there are output stage noises and input noises, the output noise sources normally being dominated by quantization noise. Quantization noise at low frequencies is relatively low and the higher frequency noise can be filtered out by a digital domain low pass filter. However, the modulation itself can be designed to provide some complex noise shaping. For example, a second order delta-sigma modulator provides two poles as opposed to the single pole of the first order, single integrator loop, the second order loops providing improved noise shaping over first order loops.
The digital filtering of the analog output is typically provided by a digital signal processor. As digital signal processors are best suited to handle digital data at much lower rates than the output samples from the modulator, the converter output rate is typically reduced by a process called decimation. Decimation is a process of digitally converting the sampling rate of the signal from a given rate to a lower rate. Typically, this decimation process is operable to remove the large amounts of high frequency quantization noise that typically are output by a delta-sigma modulator. This effectively provides the analog-to-digital converter bandwidth and allows only a tiny amount of the analog modulator's quantization noise to get through.
As a result of the various error sources and dc drift problems inherent in the analog modulator, the output of the modulator can have an offset error and a gain error. This error must be accounted for in wide dynamic range applications. Typically, some type of offset correction through the use of feedback provides the offset correction, and a gain adjustment of some type is provided for the output of the analog modulator. These typically can be quite expensive to implement in a manufacturing environment since they may require adjustment of capacitor values, etc. One type of offset correction is illustrated in Robinton, U.S. Pat. No. 4,542,354. However, this compensation is done at the modulator.